1. Field
This invention relates to improvements in ferromagnetic filters and, in particular, to nonreciprocal ferromagnetic filters intended for use in the VHF, UHF, and microwave frequency ranges.
2. Prior Art
A ferromagnetic filter, in its rudimentary form, consists of a coupling loop positioned close to a sphere of ferromagnetic material in a magnetic field. The ferromagnetic material, usually yttrium-iron-garnet, can be made to resonate at a frequency determined by the strength of the magnetic field. At microwave frequencies, these filters can have a Q ranging from 3,000 to 10,000.
Despite the high Q obtained at microwave frequencies, a ferromagnetic filter is usually not used at VHF or UHF frequencies because of the necessarily large size of the filter and the low Q achievable in these frequency ranges. Ferromagnetic filters are also usually unsatisfactory in the low frequency ranges in bandpass applications because of the high insertion loss of 1 dB or more per filter section.
Ferromagnetic material is doped to overcome a low-level power saturation occurring at low frequencies in this material. A side effect of doping is a reduction in the degree to which the material can be magnetized, making it necessary to position the coupling loops closer to the material. Unfortunately, close positioning increases the number of unwanted frequencies at which the material will resonate, making prior art ferromagnetic filters usually unsatisfactory in the VHF and the UHF ranges.
Difficulties with present ferromagnetic filters are not restricted to the VHF and the UHF frequency ranges. The most commonly available ferromagnetic filters are, for the most part, nonabsorptive, reciprocal devices. Such devices tend to be reflective and to produce high VSWR's.
Attempts have been made to overcome these difficulties by using coupling structures that produce a circularly polarized field in the ferromagnetic material. Nonreciprocal coupling can be induced in this manner, allowing for matched filter structure.
Previous methods of producing circular polarization have required the addition of components such as directional couplers and transmission lines for each filter section. The size and cost of a filter with these additional components made its implementation impractical, especially in low frequency applications.